Libyan rebel forces have been responsible for looting, arson and the abuse of civilians in their push toward Tripoli, Human Rights Watch alleged on Wednesday.
The group said it "witnessed some of these acts, interviewed witnesses to others, and spoke with a rebel commander about the abuses."
The abuses were said to have taken place in June and July -- as recently as last week -- as rebel forces pushed through the Nafusa Mountains to the south of Tripoli.
"In four towns captured by rebels in the Nafusa Mountains over the past month, rebel fighters and supporters have damaged property, burned some homes, looted from hospitals, homes, and shops, and beaten some individuals alleged to have supported government forces," said HRW.
The allegations threaten to undermine the rebels' carefully guarded image as the champions of human rights in a country that has been run by Moamer Kadhafi with an iron fist for nearly 42 years.
The revelations could also raise difficult questions for NATO countries, who have provided military support to the rebels as part of a UN-mandated mission to protect Libyan civilians.
France earlier this month admitted air dropping weapons to rebels in the Nafusa Mountains, raising the hackles of Russia and others critical of NATO's operations.
"The rebel authorities have a duty to protect civilians and their property, especially hospitals, and discipline anyone responsible for looting or other abuse," said the organisation's Joe Stork.
The group said locals reported at least one person was shot in the foot by rebel soldiers.
The rebel commander in the region, named by Human Rights Watch as Colonel El-Moktar Firnana, was quoted as admitting some abuses had taken place and that some fighters or supporters had been punished.
"If we hadn't issued directives, people would have burned these towns down to the ground," HRW quoted him as saying.
In the rebel stronghold of Benghazi, the National Transitional Council on Tuesday did not respond formally to questions about whether abuses may have taken place.
Human Rights Watch said two of the towns in question were home to a tribe close to Kadhafi.
"Al-Awaniya and Zawiyat al-Bagul are home to members of the Mesheshiya tribe, known for its loyalty to the Libyan government and Moamer Kadhafi," the group said.
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Libyan rebels in the Nafusa highlands are currently preparing to advance toward the capital, eyeing the town of Asabah as their next stop after capturing a string of hamlets.
"This will be the most important battle of the Nafusa Mountains," rebel commander Wael Brachen told AFP at the front. "This is the last town before Garyan and... it is full of armed Kadhafi supporters."
Since guerrillas took the village of Gualish on Wednesday, 17 kilometres (11 miles) from Asabah, they have been awaiting NATO air strikes that will clear their way for an advance.
French and Libyan officials meanwhile have talked up the chances of negotiating Kadhafi's withdrawal from power and an end to the conflict wracking his country, after months of military stalemate.
Kadhafi's prime minister told a French daily the embattled regime was ready to begin talks with Paris and Libyan rebels "without preconditions" and without the interference of its authoritarian "Guide".
France -- which has spearheaded the Western diplomatic and military response to the crisis -- confirmed it is in indirect talks to bring the fighting to an end and to smooth Kadhafi's departure.
"A political solution in Libya is more vital than ever and it is beginning to take shape," Prime Minister Francois Fillon told MPs before the National Assembly and later the Senate both voted to extend France's role in military action.
"The Guide will not take part in these discussions. Everything must be open," Libyan Prime Minister Baghdadi al-Mahmudi told France's Le Figaro newspaper, in an interview conducted in Tripoli and published Tuesday.
"We are ready to negotiate unconditionally," he said, although he called on NATO to halt air strikes. "We simply want a stop to the bombardments so that one can talk in a serene atmosphere. We cannot talk as bombs rain down."
A NATO spokesman however said that bombing would continue, even through the holy Muslim fasting month of Ramadan in August, so long as Kadhafi's forces attack civilians.
Mahmud Shammam, spokesman for the rebel National Transitional Council (NTC), told AFP the revolutionaries would only respond to "serious initiatives" that include the departure from power of Kadhafi and his sons.
He said Beshir Saleh, a Kadhafi ally, had approached France to propose the strongman step down but remain in Libya under international supervision, but that Kadhafi's influential son Seif Al-Islam had vetoed the idea.
In a sign of continued tensions, Libyan government spokesman Mussa Ibrahim said Tripoli regretted the French parliament vote to prolong the country's military intervention.
"We regret the vote of French elected officials. We had hoped representatives of the French people consider the situation in Libya in a realistic way, ignoring the lies of the government and media," Ibrahim told AFP.
France has previously insisted military action will continue until Kadhafi quits power but, with the costly campaign now four months old, Paris appears ready to talk.